Walnuts need sunburn protection

Brenda Carol | Mar 04, 2006

Walnut growers were reminded once again last season about how damaging the sun can be on their crop when soaring temperatures wreaked havoc on a lot of the nuts. Sunburned nuts often mold, shrivel and turn off-colors which can range anywhere from a light tan to black. Even slight deviations from the optimal white kernel can result in a drop in grade and a corresponding reduction in price to the grower.

“Early in the season last year it didn't look like we were going to have much of a problem with heat,” says Andy Dugo, pest control adviser for Mid Valley Agricultural Services in Escalon, Calif. “That didn't turn out to be the case as the season progressed.”

As temperatures soared during the mid to latter part of the 2005 season, walnut growers watched their crop and profits sizzle under the San Joaquin Valley heat.

“A grower can really get zapped on grades if he has sunburned walnuts,” Dugo says. “The difference between Class 1 and Class 2 walnuts can easily be 10 to 12 cents per pound. Then, if you go all the way down to oil stock, a grower will really take a hit. Not only do they not get paid the premium for Class 1 nuts, they don't get paid at all for the rejects.”

One option some growers have been trying to protect against sunburn is Surround, a white film of kaolin particles that is sprayed on the foliage and nuts as a wettable powder. When the product dries, it reflects infrared and ultraviolet radiation, yet has no negative impact on natural photosynthetic processes.

While there have been other sun-reflecting products on the market in the past, none can claim the same level of sunburn protection without interfering with photosynthesis. Most were basically whitewashes that “painted” tree trunks or foliage, according to Dugo.

“I've been working with the product for about three years now, primarily in walnuts,” Dugo says. “It's been very beneficial in reducing problems with heat stress and sunburn, particularly on the early varieties that are more susceptible to those kinds of problems. We see significant benefits on varieties such as Vina and Hartley. The later varieties such as Chandler benefit as well, but the differences are usually not quite as dramatic.”

Apparent at harvest

The effects of heat stress in walnuts are often not apparent until harvest. Some varieties are particularly susceptible to internal mold as a result of heat stress, but may not show outwardly visible symptoms during the growing season.

“Vina is particularly prone to internal mold problems,” Dugo says. “We tend to see a lot of rejects in that variety due to heat stress and sunburn.”

Mike Rodoani grows walnuts near Escalon, Calif. He began using Surround crop protectant primarily for sunburn protection three years ago and saw a significant reduction in “oil-less” nuts on blocks where he had applied the product.

“A lot of times you don't realize the effect the sun is having on the nuts because you can't always see it,” he says. “They look fine on the outside, but then you get to harvest and you'll have a kernel that is shriveled up to nothing. It all goes back to heat stress during the season. If you can protect against that, you can minimize rejects and inferior grades.”

The advantages of using a sunburn protectant became painfully obvious to Rodoani last season when he decided to try to get by without it. “I made the decision early in the season to skip it,” he says. “It turned out to be a very bad decision because we had some high temperatures, and that caused a lot of damage from sunburn and heat. I won't make that mistake again.”

Selling sunburn protection is not an easy job, according to Dugo. “If you have a pest out there, you can see it,” he says. “Sunburn is one of those problems that growers don't necessarily recognize as a problem until it's too late. Additionally, it takes a lot of material with a lot of water to apply it correctly and get the full benefit. That's not always an easy sell.”

Multiple applications

In spite of the hassle factor, Dugo says there are really no other competitors, at least in the walnut market. For best results Surround should be applied multiple times as a planned approach throughout the season beginning prior to a heat event.

“I make the first application of Surround about mid-June,” Rodoani says. “Then I come back about two to three weeks with a second application and a third application two to three weeks after that. The goal is to keep a uniform coating on the foliage and nuts for the best protection.”

Rodoani is very meticulous about application technique. He has a high-volume applicator and slows the tractor speed down to three miles per hour or less.

“A lot of growers don't realize what kind of coverage they're getting with their spray rigs until they start using Surround,” Dugo says. “Because this product turns white when it dries, you can see exactly what kind of coverage you're getting. Mike does a thorough job because he uses 250 gallons of water; he drives at a very slow speed and he alternates the path through the orchard when he's applying the product.”

Surround also suppresses insects such as husk fly and codling moth by creating a less desirable surface for egg lay.

“I wouldn't say that it is a stand-alone product for insect control,” Dugo says. “But it's definitely a side benefit. In organic production, that benefit is even more valuable since there are so few options available.”

Husk fly treatment

Dugo sometimes uses GF-120 Naturalyte bait, a spinosad, in combination with Surround on the organic acreage he scouts. “We've had fair results with that combination against husk fly,” he says. “We've reduced the damage to about 10 percent. In organic, that's not too bad.”

If husk fly suppression is a desired goal, Surround should be applied at the first sign of infestation in the trees or occurrence in traps. Repeat applications should be made on a 7 to 21 day schedule to maintain a uniform coating as long as the nuts are susceptible. For codling moth suppression, Surround WP crop protectant should be applied at 150 and 250 degree days after the bio-fix for each generation.

Protecting walnuts from sunburn and insect damage is a critical concern for growers in any year, but even more so when competition gets tougher. “In a year where there's a good supply of product, the processors just get pickier,” Dugo says. “If a grower can protect the quality of his crop, he's going to be in better shape than the grower who doesn't.”